1. Field of the Invention
The present invention relates to a method of continuously producing an alkylbiphenyl useful as a functional fluid such as a pressure-sensitive paper solvent, heating medium and insulating oil.
2. Description of the Related Art
Alkylbiphenyls are used as a pressure-sensitive paper solvent, heating medium, insulating oil and various solvents and various studies have been made as to a method of producing them.
There is described a method of reacting biphenyl and propylene in the presence of an aluminum chloride catalyst in the publication of Japanese Patent Application Laid-Open (JP-A) No. 49-80045. However, in the case of using a catalyst such as aluminum chloride, a water-washing and neutralizing step is required after the reaction, the step causing the generation of a large amount of waster water. Also, the equipment to be used must be constructed of acid-resistant materials. Accordingly, it is difficult to run a continuous reaction and produce alkylbiphenyls efficiently.
In the publication of JP-A No. 56-156222, there is described a method of alkylating biphenyl by using an olefin in the presence of a silica-alumina catalyst or zeolite catalyst. However, only a method is disclosed in which monoalkylbiphenyls rich in m- and p-position substituted bodies is produced in a batch system and it is therefore difficult to produce polyalkylbiphenyls such as dialkylbiphenyls and trialkylbiphenyls in a large amount efficiently.
In the publication of JP-A No. 9-40588, there is a method of producing a p-isopropylbiphenyl in a continuous flow system comprising a step of running an isomerization reaction of a m-isopropylbiphenyl and biphenyl, a step of separating a p-isopropylbiphenyl and a circulation step of returning the remainder product from which the p-isopropylbiphenyl has been separated. However, there is no description concerning a method of producing polyalkylbiphenyls such as dialkylbiphenyls and trialkylbiphenyls by using biphenyl and an olefin as starting materials.
Also, in the publication of JP-A No. 3-106833, there is a description of a method of producing a 4,4′-dialkylbiphenyl continuously, the method comprising a step of reacting biphenyls including biphenyl and monoalkylbiphenyls with an olefin or the like in the presence of an acid catalyst, a separating step of separating only a 4,4′-dialkylbiphenyl and a circulation step of returning the remainder product from which the 4,4′-dialkylbiphenyl has been separated. However, there is no disclosure of a method of producing dialkylbiphenyls and trialkylbiphenyls, which are the compounds intended in the present invention except for 4,4′-dialkylbiphenyl. Also, there is a fear that if biphenyl having a melting point as high as 70° C. exists in a high concentration, it is solidified in the circulation step. However, these problems and a method of running a continuous operation stably nowhere are disclosed. Also, 4,4′-dialkylbiphenyl has a melting point as high as 65° C., so that there is a fear that it precipitates as a crystal when cooled if it exists in a high concentration in dialkylbiphenyls. It is therefore preferable that the ratio of 4,4′-dialkylbiphenyls is small when polyalkylbiphenyls such as dialkylbiphenyls and trialkylbiphenyls are used for a pressure-sensitive paper solvent and the like.